The HP Indigo line of digital printing presses is based on digital offset color technology, which combines ink-on-paper quality with multi-color printing on a wide range of paper, foil and plastic substrates. These digital printing presses offer short-run printing, on-demand service and personalization, all at an affordable price.
Color consistency is important in digital printing presses. When a digital printing press makes many prints of an image, the first print in a run should be the same as the last print in the run, which should be the same as a print made two weeks later, which should be the same as a print made six months after that. However, colors can shift over time due to drift in the printing press. One consequence is that dots are not printed at their commanded sizes. The physical (printed) dot size is different than the digital dot size (that is, the dot size that should have been printed).
Dot gain tables can be used to improve color consistency. A dot gain table relates physical dot size to digital (commanded) dot size. During printing, the digital printing press uses dot gain tables to select or interpolate digital dot sizes that produce the dots of the proper sizes.
The dot tables can be generated in two stages. The first stage is performed to determine a proper developer voltage setting for each ink. The digital printing press prints a first set of the test patterns, and these test patterns are used to determine a developer voltage that produces the desired ink thickness. The ink thickness can be inferred from optical density at 100% print coverage. For example, a test pattern is printed at a constant developer voltage and 100% coverage, and the optical density of the test pattern is measured. If the optical density is less than desired at 100% print coverage, the developer voltage is increased, a new test pattern is printed, and optical density of the new test pattern is measured. The steps of searching for a new developer voltage, printing a new test pattern at the new developer voltage, and measuring optical density are repeated until a test pattern is printed with 100% ink coverage.
A developer voltage that produces the desired ink thickness is determined for each ink. If the digital printing press uses seven different inks, seven different developer voltages are determined. Thus the steps of searching for a new developer voltage, printing a new test pattern, and measuring optical density are performed for each ink.
Once the developer voltages are set to produce the desired ink thickness, the second stage is performed. Multiple sheets of test patterns with monochromatic swatches of uniform digital dot area are printed, and the physical dot area for each of the digital dot areas is measured. These dot areas are added to the dot gain tables.
Printing and measuring the many test patterns can be time consuming and a waste of ink and print substrates. A faster, less expensive, more efficient method of generating the dot gain tables is desirable.